1. Field of the Invention
This invention relates to a stabilized polyolefin resin composition. More particularly, it relates to a polyolefin resin composition having improved resistances to oxidation and to weather which is prepared by adding a phenol antioxidant, a sulfur antioxidant, a hindered amine light stabilizer and an aliphatic monocarboxylate of a metal belonging to the group Ia or IIb to a polyolefin resin containing an inorganic filler.
2. Description of the Prior Art
It is known that a polyolefin resin such as polyethylene or polypropylene is deteriorated by, for example, heat, light, oxygen or nitrogen oxide and consequently becomes inavailable any more since it is colored or its mechanical strengths are lowered.
In order to prevent such deterioration of a polyolefin resin, there have been applied a number of additives either alone or as various mixtures.
Among these additives, a phenol antioxidant is widely employed as a basic stabilizer. However it can exert only a limited effect when employed alone. Thus it is combined with other additive(s) depending on the purpose of the application. For example, it is combined with a sulfur antioxidant such as a dialkyl thiodipropionate in order to achieve a high heat resistance. Further it is combined with various light stabilizers in order to achieve a high weather resistance. Furthermore it is combined with both of a sulfur antioxidant and a light stabilizer in order to achieve high resistances to heat as well as to weather.
Examples of the light stabilizer to be combined with a phenol antioxidant include benzophenone, benzotriazole and benzoate UV absorbers and hindered amine compounds. In particular, the application of hindered amine light stabilizers such as a polyalkylpiperidine has been more and more increasing since they are noncoloring, exert a high photostabilizing effect and improve the heat resistance.
However it is known that the combined use of a hindered amine light stabilizer and a sulfur antioxidant would offset the effects with each other. In this case, the weather resistance of the obtained product would be particularly lowered. Thus a composition containing a hindered amine light stabilizer is generally free from any sulfur anitoxidant.
A carboxylate of a metal belonging to the group IIa, such as calcium stearate, has been commonly used in order to neutralize a residual catalyst in a polyolefin resin or to improve the lubricity thereof, though this additive would scarcely affect the resistance to heat or to weather thereof.
An inorganic filler such as talc or calcium carbonate is added to a polyolefin resin in order to improve the stiffness, mechanical properties and dimentional accuracy thereof. A polyolefin resin containing an inorganic filler is generally used in a product which should have a high weather resistance, for example, an automotive part or an electrical appliance to be used outdoor or under a fluorescent lamp. Therefore an inorganic filler should be combined with a light stabilizer. A hindered amine light stabilizer is commonly employed therefor since it is noncoloring and can exert a high stabilizing effect.
However the effect of an antioxidant on a polyolefin resin containing an inorganic filler is significantly lower than that on a polyolefin resin free from any inorganic filler. Accordingly a two-component system comprising a phenol antioxidant and a hindered amine light stabilizer cannot give any sufficient heat resistance. Thus it is required to further add a sulfur antioxidant thereto in order to achieve a satisfactory heat resistance. As described above, however, the combined use of a hindered amine light stabilizer and a sulfur antioxidant would significantly lower the effect of the hindered amine light stabilizer in improving the weather resistance. Namely, it is impossible to satisfactorily improve both of the resistances to weather and to heat by this system.
Accordingly there has been urgently required to develop a polyolefin resin composition containing an inorganic filler wherein the combined use of a phenol antioxidant, a sulfur antioxidant and a hindered amine light stabilizer would lower none of the effects of these additives.